Certain inorganic materials have been used in electrical wire and cable constructions to provide high temperature resistance and/or high voltage ratings for many years with mica (a silicate mineral) being perhaps the most commonly used.
Mica is generally found as platlets or flakes that are extremely fragile and are sometimes pulverized into a powder for particular uses. Because of such fragility, mica flakes or platlets have been heretofore fabricated into a "mica paper" that is bonded to a flexible backing such as a high temperature resistant fibrous material like fiberglass or paper tape by means of a high temperature resistant adhesive such as a curable silicone based adhesive. The use of such backing tapes enables mica to be applied to electrical wire and cable products in a manufacturing setting. Unfortunately, mica, even though bonded to such tapes, tends to crack upon being wrapped or folded about smaller diameter conductors or when the finished product is flexed or bent.
Another disadvantage associated with mica tapes is that they are not generally commercially available in long lengths resulting in costly and time consuming stoppages for splicing tape ends together in order to make long length electrical wire or cable products which in itself creates unevenness in the region of the splices.
Another disadvantage with the use of such mica backed tapes is that the adhesives bonding the mica to the fibrous material characteristically harden when subjected to heat causing the insulated wire or cable to stiffen considerably.
An example of the use of mica backing tapes in the windings of an electrical coil is disclosed in U.S. Pat. No. 4,204,181, the disclosure of which is incorporated herein by reference.
Although powdered mica has been used in electrical products in the past, it has always been blended into a binder or adhesive of some type that are characteristically curable and tend to harden and stiffen when subjected to heat as in the case of the mica backed tapes. An example of the use of powdered mica in a curable binder for an electrical article is disclosed in U.S. Pat. No. 3,930,915, the disclosure of which is incorporated herein by reference.
Even where flaked mica has been electro-deposited upon a conductor in the past, it has been in combination with a curable resin which again tends to harden and stiffen when subjected to heat and of which examples are disclosed in U.S. Pat. Nos. 4,723,083 and 4,724,345, the disclosures of which are incorporated herein by reference.
The present invention eliminates the heretofore described disadvantages associated with the use of mica backed tape and binder/powdered mica in wire an cable products by providing a free-flowing mica about the conductor to provide a high temperature resistant product that remains flexible when subjected to heat.
Another advantage that arises is that when the fibrous material in close proximity to the electrical conductor is fiberglass, the strippability of the fiberglass from the conductor can be enhanced by treating the fiberglass with a solution containing sodium silicate and then heat aging the treated material as disclosed in my co-pending application Ser. No. 08/036,382 entitled "Strippable Fiberglass Insulated Conductor", filed on the same date as this application.